1. Field of the Invention
The invention relates generally to satellite direction determining systems and more particularly to a direction determining system for a Global Positioning System (GPS) having a linear antenna array, a GPS receiver and a switching facility for determining the pointing angle of the linear antenna array.
2. Description of the Prior Art
In the field of satellite navigation, the pointing angle of a linear antenna array relative to the position of a satellite of a Global Positioning System (GPS) has been determined as is described in U.S. Pat. No. 4,384,293. In the Global Positioning System (GPS), the satellite position is known and the location of the direction determining facility is determined with a GPS receiver as described in published German patent DE-A-34 27 058 which corresponds to commonly assigned copending application Ser. No. 758,191 filed on July 23, 1985. From this data, various types of information can be determined such as elevation, azimuth, spatial orientation of the vehicle or aircraft containing the direction determining facility. The number of linear antenna arrays provided is dependent on whether the direction is to be determined in one or more planes.
In the prior art direction determining facility, the linear antenna array consists of a plurality of antennas in which output signals are switched from one antenna to the next such that the locking of the phase-locked loop of the GPS receiver to the received carrier signal is preserved. The above cited U.S. Pat. No. 4,384,293 also mentioned that only two antennas can be employed and in such a case, GPS carrier signals of different frequency are evaluated. The major problem still existing in the prior art is the speed at which the output signals can be switched from one antenna element to the next. It is important that the phases of the carrier signal received at the two antennas can be measured nearly simultaneously. It is the differences in the phases of the carrier signals that are employed in determining the direction of the vehicle or aircraft. Measuring the phases of the carrier signal received at the two antennas nearly simultaneously eliminates errors caused by a movement of the direction-determining facility between the two measurements. The same principle applies to any frequency and phase drift of components which are important for measurement accuracy.